Device and method for heat treatment of steels, including a wet cooling

ABSTRACT

The invention relates to a method and a device for rapidly cooling a metal strip and removing residues present on the strip after this cooling, wherein the residues are formed during a cooling of said metal strip by a non-oxidizing liquid solution for the metal strip and a stripping liquid solution for the oxides present on the surface of the strip, or by a mixture of this liquid solution and a gas.

The invention relates to annealing lines and to the hot dip galvanizinglines for flat products, and more particularly to continuous linesequipped with a non-oxidizing and stripping liquid cooling section. Itis associated with the field of metallurgy, and relates both to heattreatment and to the chemistry of steels.

Technical Problems Addressed by the Invention

The lines equipped with gas cooling do not make it possible to cover allsteels having a high elastic limit, on account of an insufficientcooling gradient. Indeed, the gas cooling, typically performed byhigh-speed blowing, onto the product, a mixture of nitrogen andhydrogen, makes it possible to achieve cooling speeds of up to 200° C./sfor steel strips of 1 mm thickness. This is insufficient with respect tothe gradients sought for obtaining the desired metallurgical structureof new generation steels having a high elastic limit, in particularmartensitic steels, which typically require cooling speeds of between500° C./s and 1000° C./s, indeed 2000° C./s, for steel strips of 1 mmthickness.

In order to achieve a cooling gradient that is sufficient for thethermal cycles of new generation steels, it is necessary to pass througha step of liquid quenching, by spraying a liquid, or a mixture of aliquid and a gas, for example nitrogen or a mixture of nitrogen andhydrogen, onto the strip. The flow rates and pressures to be used in thecooling section depend on the type of the steels to be treated and thecooling gradients to be complied with. The temperature of the strip atthe output of the cooling section is typically between 50° C. and 500°C.

However, the water cooling causes surface oxidation which is oftenincompatible with the subsequent coating when it is present in a largequantity. Said oxidation takes the form of FeO, Fe₂O₃, and Fe₃O₄ for atemperature of said product of above 575° C., and it takes the form ofFe₂O₃ and Fe₃O₄ for a product temperature of below 575° C. Theimplementation of an intermediate chemical stripping section between thecooling and the coating becomes necessary from then on. Said chemicalstripping section is costly in terms of investment and operation, and itincreases the footprint of the installation.

FR3014447 and FR3064279 by the applicant describe methods for liquidquenching, the cooling liquid of which is non-oxidizing for the stripand is stripping with respect to the oxides present at the surface ofthe strip, in particular those formed from addition elements containedin the steel to be treated. This liquid is, for example, made up of amixture of demineralized water and formic acid. It may be sprayed ontothe strip by means of nozzles, alone or together with a gas, for examplenitrogen or a mixture of nitrogen and hydrogen. These methods haveproven their effectiveness for preventing or reducing the presence ofoxides at the surface of the product, by limiting their formation and/orby directly stripping those which have been able to form at the surfaceof the strip. It is therefore no longer necessary for an intermediatestripping section to be implemented.

However, the applicant has noted that the use of a non-oxidizing andstripping cooling liquid leads to the formation of residues, for exampleformate salts or iron hydroxides, which remain present on the strip. Onan annealing line, said residues provide coloring to the strip at theline output. They may pose problems for the subsequent use of the sheetmetal, in particular for the phosphating treatment for the productsintended for the automotive market. The phosphating is the first step ofa painting process. The sheet metal undergoes this treatment in orderfor the layers of paint applied to subsequently adhere correctly andover a long duration. If the phosphating has faults, there is a risk ofsubsequent detachment/peeling/corrosion during the use of the vehicle.The condition for good phosphating is that the sheet metal should beperfectly clean, without pollution of any kind. On a dip galvanizingline, said residues may be the origin of adhesion faults of the coating,and make this incompatible with the quality level sought, in particularin accordance with the requirements of sheet metal for motor vehicles.Said residues of non-oxidizing and stripping cooling liquid must thus beremoved in order for the surface quality of the strip at the output ofthe line to be in accordance with the clients' expectations.

Furthermore, the addition elements present in the new generations ofsteel can oxidize very easily, compared with iron, and pollute thesurface of the product, making it incompatible with galvanizationbecause they prevent the good adhesion of the coating. It is thuspossible to find the presence of MnO_(x), SiO_(x), BO_(X), Mn₂SiO₄,MnAl₂O₄ and MnB₂O₄ at the surface of the product, even when the furnaceatmosphere has a very low dew point, for example of −40° C. Unlike ironoxide, these oxides are not reduced under the atmosphere present in thefurnace. This is made up of a mixture of nitrogen and hydrogen,typically having 4% hydrogen. The addition of pre-oxidation during theheating phase makes it possible to limit the presence of these oxides atthe surface of the strip. Said pre-oxidation is, for example, performedin a direct flame preheating section (DFF—Direct Firing Furnace), bymeans of excess air-driven burners. It may also be performed in aradiant tube heating section (RTF—Radiant Tube Furnace), for example ina dedicated chamber having an oxidizing atmosphere made up of a mixtureof N₂ and O₂ or indeed N₂ and H₂O, or by another oxidizing atmosphere.During said pre-oxidation, a barrier of iron oxide forms at the surface,preventing the migration of addition elements towards the surface; theoxygen diffuses in the matrix and oxidizes these elements and thusblocks them in the steel. The iron oxide is then annealed in thesections downstream of the furnace, under a reductive atmosphere. Onlythe oxides of addition elements initially present at the surface, and alimited portion of those which have been able to migrate towards thesurface, are thus present at the surface of the product.

For some steel grades, it is advantageous to perform selective internaloxidation in a preheating or heating section. It is distinguished fromthe pre-oxidation in that it targets only the addition elements. It isobtained by combining, at depth, oxygen atoms originating from thesurface, with some atoms of addition elements, leading to the formationof oxide precipitates. The selective internal oxidation is generallyperformed in a dedicated chamber which is sufficiently oxidizing tooxidize the addition elements but not the iron.

Said pre-oxidation or said selective internal oxidation, associated withwater cooling, is not of great interest because, even if it limits theformation of oxides at the surface from addition elements during theheating, the liquid water cooling which follows will generate ironoxides at the surface which are incompatible with the dip galvanizingmethod. In this case, it would be necessary to add a chemical strippingsection before the coating.

In summary, the combination of a pre-oxidation of the product, orselective internal oxidation, and humid non-oxidizing and strippingcooling makes it possible to prevent the main disadvantages mentionedabove. However, it may lead to the formation of residues at the surfaceof the product, and cause a lower surface quality, for example adhesionfaults of a subsequent coating. The invention makes it possible to limitthe formation of said residues and to treat/eliminate those which havebeen formed and which are present at the surface of the strip after theliquid cooling.

TECHNICAL BACKGROUND

The applicant does not know of any solution according to the prior artwhich addresses the reduction of the formation of said residues or theirremoval, in particular because the cooling of the strip by anon-oxidizing and stripping liquid is not yet made use of on industriallines, in production.

Following the cooling, steel grades require overaging. This has the roleof making the steel undergo aging in order to make it pass from a stateof metallurgical disequilibrium at the cooling outlet into a stablestate. It is obtained by keeping the strip at a given temperature for asufficient time. The overaging temperature, generally between 300° C.and 600° C. depending on the steel grade, is an important processparameter to be adhered to. The time for maintenance at the overagingtemperature is typically between 15 seconds and 90 seconds, depending onthe steel grade.

Following the cooling, it is expedient not to exceed a temperature,above the overaging temperature, which would bring about an undesiredmetallurgical transformation of the steel, which would risk cancellingout the metallurgical effect of the dipping, and deterioration of themechanical properties of the strip. An overaging chamber may comprise aheating means for the strip, in order to bring said strip to theoveraging temperature. In a variant, said heating means may bepositioned upstream of the overaging temperature. Said heating means maybe an inductor for rapidly bringing the strip to the requiredtemperature. The overaging chamber furthermore comprises radiant heatingelements (spark plugs, radiant tubes, or electrical tapes) which ensuretemperature maintenance of the strip. An overaging chamber is kept undera hydrogenated atmosphere made up of a mixture of nitrogen and hydrogen,traditionally comprising approximately 4% hydrogen, by volume. Saidhydrogen content may not be sufficient for reducing the residues presenton the strip at the typical overaging conditions, or it may require theoveraging temperature to be increased or the dwell time at the overagingtemperature to be lengthened.

In an annealing line, at the output of the overaging section, the stripis cooled to ambient temperature. In a galvanizing line, at the outputof the overaging section, the strip may be heated or cooled in order tobring it to a coating temperature, depending on whether the overagingtemperature is lower or higher than the coating temperature. This may beperformed by dipping, i.e. by immersing the strip into a bath containingthe metal, or the metal alloy, forming the coating to be applied to thestrip, or by any other means. The coating may be zinc, an alloycontaining zinc, or of any other kind. For a dip coating, the coatingtemperature is close to that of the bath of metal liquid in which it isimmersed.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a method for rapid wetcooling a metal strip, making it possible to limit the amount ofresidues present on a metal strip at the output of a rapid coolingsection of a continuous line, is proposed, said residues being formedduring a cooling of said metal strip by a liquid solution that isnon-oxidizing for the metal strip and stripping for the oxides presentat the surface of the strip, or by a mixture of this liquid solution anda gas. The method making it possible to limit the amount of residuepresent on the strip is characterized in that it comprises a first stepcomprising rapid water cooling, followed by a second step of rapid wetcooling, by a solution that is non-oxidizing for the strip.

Since the residues are formed during the rapid cooling by the solutionthat is non-oxidizing for the strip, it is advantageous to reduce thesignificance thereof while performing the first step of water cooling.Moreover, during the first step of water cooling, the contact betweenthe water and the strip is limited on account of the presence of a vaporfilm at the surface of the strip, which has the effect of limiting theoxidation of the strip.

According to a second aspect of the invention, a method for removingresidues present on a metal strip at the output of a cooling section ofa continuous line is proposed, the residues being formed during coolingof said metal strip by a liquid solution that is non-oxidizing for themetal strip and is stripping for the oxides present at the surface ofthe strip, or by a mixture of said liquid solution and a gas, in orderto make the surface of the strip compatible with the subsequent methods(galvanization, phosphating, electro-galvanizing, etc.).

The method according to the invention comprises a step of removingresidues obtained by reduction of the residues using hydrogen.

By “step for removing the residues,” the present invention means areduction step of an oxidoreduction reaction between an oxidant and areducing agent.

Preferably, the first step of cooling cools the strip to a temperatureof greater than or substantially equal to the Leidenfrost temperature.For example, the temperature is between the Leidenfrost temperature andsaid temperature increased by 50° C.

Preferably, the second step of cooling cools the strip from atemperature of less than or equal to the Leidenfrost temperature. Thisfeature allows for effective stripping of the surface of the strip, incombination with the end of the cooling.

The step of reducing the residues may be of a duration of between 15seconds and 300 seconds for a strip temperature of between 50° C. and600° C.

The step of reducing the residues may be performed under the hydrogencontent present in the furnace atmosphere, i.e. without increasing it.Thus, for example, the hydrogen content may remain at 4%, which is acontent often used on the existing lines. In order to achieve effectivereduction of residues, it is thus necessary either to lengthen the stepof reduction of the residues, or to increase the temperature of thestrip during said step, or a combination of the two, compared with areduction of oxides which would be performed under a higher hydrogencontent.

Advantageously, the step of reduction of residues is performed when themetal strip is in an atmosphere of which the hydrogen content is between5% and 100%, and preferably greater than or equal to 10%, by volume.

For the step of reduction of residues, hydrogen, or a hydrogenatedatmosphere of which the hydrogen content is between 5% and 100%, andpreferably greater than or equal to 10%, by volume, may be blown ontothe metal strip.

The hydrogen, or the hydrogenated atmosphere, blown onto the metalstrip, may be of a temperature of between 500° C. and 800° C. Said hightemperature of the blown gas allows for greater effectiveness of thestep of reducing residues compared with that obtained by blowing thehydrogen, or the hydrogenated atmosphere, at a lower temperature. Theduration of the reduction step, and/or the temperature of the stripduring said step, can thus be reduced.

According to one possibility, the speed of blowing of the hydrogen, orof the hydrogenated atmosphere, is between 10 m/s and 160 m/s uponcontact with the metal strip.

The method for removing residues according to the first aspect of theinvention may further comprise a step of pre-oxidation, or of selectiveinternal oxidation, of the surface of the strip, performed in apreheating, heating or temperature maintenance section of the metalstrip, arranged in front of the cooling section.

The method for removing residues according to the first aspect of theinvention may be implemented on a continuous line having a section fordip coating of the metal strip in a molten bath, and may furthercomprise, after the step of reduction of residues, a step of heating orcooling of the metal strip to a temperature close to the temperature ofthe bath.

The hydrogenated atmosphere is for example made up of a mixture ofnitrogen and hydrogen.

According to a third aspect of the invention, a use of a continuoustreatment line for a metal strip is proposed, comprising a first step ofrapid cooling of said metal strip by means of water, or a mixture ofwater and a gas, and a second step of rapid cooling of the strip bymeans of a liquid solution which is non-oxidizing for the metal stripand stripping for the oxides present at the surface of the strip, orusing a mixture of said liquid solution and a gas, the use furthercomprising a step of reducing residues by means of hydrogen, saidresidues being formed during the rapid cooling of the strip.

According to a fourth aspect of the invention, a continuous treatmentline for a metal strip is proposed, comprising a section for rapidcooling of the metal strip by projecting thereon a liquid solution, or amixture of a liquid solution and a gas, comprising a first zone in whichthe liquid solution is water, followed downstream, in the direction oftravel of the strip, by a second zone in which the liquid solution isnon-oxidizing for the metal strip and stripping for the oxides presentat the surface of the metal strip, the line further comprising,downstream of said rapid cooling section in the direction of travel ofthe strip, a section for reduction of residues formed during the coolingand present on the strip, said reduction section being designed toimplement a removal method according to an aspect of the invention orone or more of the developments thereof.

The reduction section may comprise means for reducing residues by meansof hydrogen, comprising a means for blowing hydrogen, or a hydrogenatedatmosphere, onto the metal strip in order to expose the metal strip toan atmosphere of which the hydrogen content is between 5% and 100% byvolume, and to a temperature of between 500° C. and 600° C.

The section for reducing residues may comprise, at the inlet in thedirection of travel of the strip, a rapid heating device for bringingthe strip to a temperature close or equal to a predetermined temperatureat which chemical reactions for reducing residues start.

Said rapid heating device is necessary when the power of the means forheating the strip, present in this section, does not make it possible torapidly reach this temperature. Indeed, if the power density of theheating means is low, the time required for reaching the temperaturenecessary for the reduction step has to be added to the dwell time ofthe strip in the section. On a new line, this would lead to extendingthe length of the section, and on an existing line this would lead toreducing the speed of the strip.

The rapid heating means also makes it possible to limit the time duringwhich the strip is brought to a temperature higher than that requiredfor obtaining the sought metallurgy, thus limiting the undesiredmetallurgical transformations.

According to one possibility, the section for reducing residues formspart of an overaging section.

The section for reduction of residues may comprise a means for blowinghydrogen, or a hydrogenated atmosphere, onto the metal strip.

According to one embodiment, the line according to the fourth aspect ofthe invention may further comprise a chamber for pre-oxidation, orselective internal oxidation, of the surface of the strip arranged in apreheating section, a heating section, or a temperature maintenancesection, of the metal strip, said section being positioned upstream ofthe rapid cooling section, in the direction of travel of the strip.

According to another aspect of the invention, a computer program productis proposed, comprising instructions which cause a line according to thefourth aspect of the invention, or one or more of the developmentsthereof, to execute the steps of a method according to the first aspectof the invention, or one or more of the developments thereof.

Advantageously, the step of reduction of residues is performed when thestrip is in an atmosphere of which the hydrogen content is greater thanor equal to 6%, advantageously greater than or equal to 7%,advantageously greater than or equal to 8%, advantageously greater thanor equal to 9%, advantageously greater than or equal to 10%, by volume.

According to an embodiment of the invention, the thermal andmetallurgical cycle of the strip may comprise one or more of thefollowing steps:

Preheating and pre-oxidation of the strip, performed in a direct flamesection DFF, the pre-oxidation being intended to form a layer of ironoxide at the surface of the strip, and thus to limit the quantity ofoxides of addition elements present at the surface of the strip upstreamof the rapid cooling.

Heating and temperature maintenance in two radiant tube sections RTF,under a reductive atmosphere of nitrogen and hydrogen, in order toobtain the desired metallurgy prior to the rapid cooling, in particularthe desired austenite proportion. During said heating and saidmaintenance, the iron oxides present at the surface of the strip areprogressively reduced by the hydrogen. As long as the layer of ironoxide is not completely reduced, it prevents the migration of additionelements towards the surface of the strip. It is therefore advantageousfor the layer of iron oxides to not be totally removed until the end ofthe maintenance, before the start of the rapid cooling. If iron oxidesremain at the end of the maintenance, these will be stripped during thecooling. However, the presence thereof at the end of maintenance is notdesired, because the stripping products would pollute the solutionsprayed for the cooling. It would thus be necessary to replace it moreoften, leading to increased consumption of acid and demineralized water.The oxides formed from addition elements are not, themselves, reduced inthe RTF.

Dipping using a non-oxidizing and stripping fluid, in a cooling section,in order to obtain the desired metallurgy, in particular thetransformation of a portion of the austentite into martensite. Saidnon-oxidizing dipping strips, at the surface, the oxides which could bedamaging to the quality of the galvanization, but leaves residues on thestrip.

Reheating the strip to a temperature close to the starting temperatureof the reactions for reducing residues present on the strip, in aninduction heating section.

At the inlet of the overaging section, blowing of hydrogen, or of ahydrogenated atmosphere, onto the strip in order to bring itstemperature to that required for starting the reactions for reducingresidues.

Cooling of the strip to the overaging temperature.

Maintenance at the overaging temperature in order to set themetallurgical structure, during which the surface of the strip iscleaned (the residues formed during the dipping and present at thesurface of the strip at the output of dipping are eliminated).

For an annealing line, the thermal cycle then comprises cooling of thestrip to the ambient temperature.

For a dip galvanizing line, the thermal cycle then comprises:

Reheating or cooling the strip to a coating temperature in an inductionheating section or a gaseous cooling section.

Coating of the strip by hot immersion in a zinc bath.

Final cooling of the strip in a cooling tower.

Examples of reactions between the hydrogen and the residues implementedaccording to the method of the invention are set out below, for the caseof manganese:

(HCOO)₂Mn+H₂→CO+CO₂+H₂O+Mn

(HCOO)₂Mn+H₂→2HCOOH+Mn

HCOOH+H₂→H₂CO+H₂O (formaldehyde)

H₂CO+H₂→CH₃OH (methanol)

Similar reactions result for iron and other addition elements.

The effectiveness of the reaction is determined in particular by thefilm temperature at the surface of the strip, the hydrogen content, thedew point of the atmosphere, the duration of contact between thereagents, and the flow speed of the hydrogen, or of the hydrogenatedatmosphere, at the surface of the strip.

In order to simplify the description of the invention, in the followingit will be considered that the removal of residues is performed in anoveraging section. It may also be performed in a section dedicated tothis function, in particular if the line does not comprise an overagingsection.

According to an embodiment of the invention on a line having a largeoveraging section, and/or when the overaging is performed at a hightemperature, the method according to the invention is implemented insaid overaging section by placing this under a hydrogen-enrichedatmosphere. Said atmosphere has a volumetric hydrogen content of between5% and 100%, depending on the dwell time and the temperature of thestrip in the overaging section. The hydrogen content is preferablygreater than or equal to 10%. The entirety of the overaging section maybe kept at this higher concentration of hydrogen, or just a portionthereof may be, depending on the dwell time of the strip required atthis higher concentration for eliminating residues. In thisconfiguration, it is not necessary to provide a particular device forinjecting hydrogen, other than those typically present on this section.This solution is possible when the overaging temperature is sufficientfor starting the chemical reactions of residue reduction, and when thedwell time in the section is sufficient for having available the timerequired for eliminating the residues. This embodiment of the inventionis thus limited to lines comprising a large overaging section and/or tosteel grades which allow for a high overaging temperature. It is equallyapplicable when the temperature at the end of cooling is equal to theoveraging temperature, as shown in FIG. 4 , or when said temperature isbelow the overaging temperature, as shown in FIG. 5 . When it is lower,the strip is first brought to the overaging temperature, for example byinduction heating.

According to another embodiment of the invention, in the case where theoveraging temperature is not sufficient for starting the chemicalreactions for residue reduction, the strip, or the film at the surfaceof the strip, is brought to a temperature sufficient for starting thechemical reactions, before bringing it, or returning it, to theoveraging temperature. Advantageously, the strip is brought to atemperature that is sufficient for starting the chemical reactions atthe inlet of the overaging section. Said higher temperature and thepossible dwell time at this temperature are limited to those requiredfor starting the chemical reactions, so as not to influence themetallurgy and the mechanical properties of the strip. As for theprevious embodiment, the hydrogen content is increased in the overagingsection in order to promote the reduction of the residues.

As shown in FIGS. 4 and 6 , when the metallurgy of the steel does notrequire cooling of the strip to below the temperature required forstarting the chemical reactions for reduction of residues, the coolingof the strip is advantageously stopped at this temperature. As shown inFIGS. 5 and 7 to 9 , when the metallurgy of the steel requires coolingof the strip to below the temperature required for starting the chemicalreactions for reduction of residues, it is first necessary to bring thestrip, or the film at the surface of the strip, to this temperature. Aswill be seen in the following, this return to the temperature necessaryfor starting the chemical reactions may be achieved using equipmentaccording to the prior art or by dedicated equipment according to theinvention. This can be achieved progressively or in stages. It may befaster or slower, depending on the type of heating applied.

In the remainder of the description of the invention, two examples ofheating means for bringing the strip, or the film at the surface of thestrip, to the temperature required for starting the chemical reactionsof residue reduction will be described. Other means which are notdescribed may be used.

A first heating means for bringing the strip to the temperature requiredfor starting the chemical reactions is induction heating. It has theadvantage of allowing for a high power density for a rapid increase intemperature.

A second heating means for bringing the strip to the temperaturerequired for starting the chemical reactions is convection heating. Itconsists in blowing, onto the strip, hydrogen, or a hydrogenatedatmosphere having a hydrogen content of between 5% and 100%, andpreferably greater than 10%, and at a high temperature, for example 800°C. This solution makes it possible to rapidly reach a temperature of thefilm at the surface of the strip that is sufficient for starting thechemical reactions, without it being necessary to bring the entirethickness of the strip to this temperature. The mixing of the atmosphereclose to the surface of the strip also makes it possible to acceleratethe chemical reactions.

In a variant, the return to the temperature required for starting thechemical reactions is performed in two steps, the first for example byheating by induction, the second by heating by blowing, as describedabove.

The blowing device may, for example, comprise nozzles, slits, tubes, orplates comprising holes. In order to simplify the description of theinvention, in the following only the case of nozzles will be discussed,without this being in any way restrictive.

The blowing device is advantageously positioned at the inlet of theoveraging section. It can nonetheless be positioned at a point of thesection for which the remaining length downstream in the overagingsection allows for a sufficient dwell time for eliminating the residues.The blowing device may comprise a plurality of gas jets over the widthof the strip and on each of the two large surfaces of the strip. Thejets may be placed so as to face one another, or so as to be offset overthe width and/or the length of the strip.

The pitch between two nozzles of the same row may be selected accordingto the opening angle of the jet and the distance between the nozzles andthe strip, so as to cover the entire width of the strip while limitingthe coverage between the jets. It may typically be between 50 mm and 200mm.

Depending on the maximum speed of travel of the strip, a plurality ofrows of nozzles may be placed on each face of the strip. The pitchbetween two rows of nozzles is defined according to the maximum speed oftravel of the strip. It may typically be between 50 and 200 mm.

The jets may be substantially perpendicular to the strip, or they areinclined at an angle which may be between 1° and 45° in the flowdirection of the strip (downstream) or in the direction contrary to thetravel of the strip (upstream).

Preferably, the distance from the blowing orifices to the strip istypically between 40 mm and 200 mm.

The gas feed rate of the nozzles may be controlled individually per rowof nozzles, per pair of rows of nozzles (the pair comprising two rowspositioned so as to be substantially facing one another on either sideof the strip), or according to any other configuration.

Preferably, the speed of escape of gas from the nozzles is between 10m/s and 160 m/s, and preferably between 80 m/s and 130 m/s.

The amount of gas blown onto the strip may be controlled in particularaccording to the temperature thereof, the hydrogen content thereof, thespeed of travel of the strip, and the film temperature of the strip. Itmay typically be between 0.5 and 15 kg/m² of strip, for example 1 and 3kg/m² of strip for a gas of 10% hydrogen, a mixing temperature of 800°C., a strip of width 1 m travelling at 100 m/min, and a dwell time inthe overaging section of 30 s.

The nozzles may be supplied with a gas of which the temperature istypically between 500° C. and 800° C. The concentration of hydrogen inthe gas blown onto the strip may in particular be associated with thetemperature thereof and the blowing speed thereof, as well as with thefilm temperature sought on the strip. It will be lower, the higher thetemperature levels of the gas and the blowing speed.

The temperature of the gas blown onto the strip brings about a heatexchange therewith, mainly by convection. The blowing parameters, forexample the temperature of the gas, may be controlled such that theinput of heat to the strip by the blown gas is limited to the amount ofheat necessary to bring and/or maintain the strip, or the film at thesurface of the strip, to/at the desired temperature. The heating powerof the heating devices at the input of the overaging section, orupstream thereof, is controlled such that they supply the additionalamount of heat which may be necessary for bringing, or maintaining, thestrip to or at the desired temperature.

The gas blown onto the strip can be recirculated, using a recirculationfan, the suction side of which is connected to the overaging section,and the discharge side of which is connected to the supply of thenozzles. The recirculation circuit may comprise means for controllingthe temperature of the gas (heating or cooling) in order to bring thegas to the temperature desired at the exhaust of the nozzles. A partialrenewal of the atmosphere of the overaging section may be performedcontinuously in order to preserve the desired concentration of hydrogenat the exhaust of the nozzles. The device may also comprise at least oneinjection point for new gas into the section, it being possible for saidgas to have the hydrogen concentration desired at the exhaust of thenozzles, or a higher concentration which may be up to 100% hydrogen.

The hydrogen content of the overaging section, and/or that of the gasoptionally blown onto the strip, is also selected according to the typeof steel to be treated and the coating quality sought. The hydrogencontent may be reduced depending on the amount of residual residuestolerated.

Depending on the type and the content of additional elements present inthe steel to be treated, it may be necessary to add a pre-oxidationstep, or selective internal oxidation, in one of the heating sectionsupstream of the cooling, as described above.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the invention will become clear fromreading the following description, for the understanding of whichreference will be made to the accompanying drawings, in which:

FIG. 1 is a longitudinal schematic partial view of a shaft furnacegalvanizing line according to an embodiment of the invention;

FIG. 2 is an enlarged view of the rapid cooling section of FIG. 1 ;

FIG. 3 is an enlarged view of the rapid cooling section according to avariant of the invention;

FIG. 4 is a graph schematically showing the variation in the exchangecoefficient at the surface of the strip, as a function of thetemperature of the strip, during the rapid cooling thereof;

FIG. 5 is an enlarged view of the sections 105 for induction heating and106 overaging of FIG. 1 ;

FIG. 6 is a graph showing the temperature of the strip as a function oftime according to a first embodiment of the method according to theinvention;

FIG. 7 is a graph showing the temperature of the strip as a function oftime according to a second embodiment of the method according to theinvention;

FIG. 8 is a graph showing the temperature of the strip as a function oftime according to a third embodiment of the method according to theinvention;

FIG. 9 is a graph showing the temperature of the strip as a function oftime according to a fourth embodiment of the method according to theinvention;

FIG. 10 is a graph showing the temperature of the strip as a function oftime according to a fifth embodiment of the method according to theinvention;

FIG. 11 is a graph showing the temperature of the strip as a function oftime according to a sixth embodiment of the method according to theinvention;

FIG. 12 is a graph showing, for a galvanizing line, the temperature ofthe strip as a function of time for 3 different overaging temperaturesaccording to the first embodiment of the method according to theinvention shown in FIG. 8 .

DETAILED DESCRIPTION OF THE INVENTION

Since the embodiments described in the following are in no way limiting,it is in particular possible to envisage variants of the invention thatcomprise only a selection of features described in the following, in amanner isolated from the other features described, if this selection offeatures is sufficient for providing a technical advantage or fordistinguishing the invention from the prior art. This selectioncomprises at least one feature, preferably functional and withoutstructural details, or having some of the structural details if thispart alone is sufficient for providing a technical advantage or fordistinguishing the invention from the prior art.

In the description which follows, elements having an identical structureor analogous functions are denoted by the same reference signs.

With reference to the schematic view of FIG. 1 of the accompanyingdrawings, a longitudinal schematic partial view of a shaft furnacegalvanizing line according to an embodiment of the invention, in which ametal strip 1 circulates, is visible. It comprises, successively and inthe direction of travel of the strip, a section 100 for direct flamepreheating in which pre-oxidation of the strip is carried out, a heatingsection 101, a maintenance section 102, a section 103 for slow gaseouscooling, a section 104 for rapid cooling using an aqueous liquidsolution, a section 105 for induction heating, an overaging section 106,a furnace outlet section 107, and a dip galvanizing section 108.

With reference to the schematic drawing of FIG. 4 of the accompanyingdrawings, a graph schematically showing the variation in the exchangecoefficient at the surface of the strip, as a function of thetemperature of the strip, during the rapid cooling thereof in thesection 104, is visible. The X-axis shows the temperature of the strip,and the Y-axis shows the exchange coefficient. In this graph, thedevelopment of the exchange coefficient during the cooling of the stripis read from right to left. Until reaching the temperature denoted bythe letter L on the X-axis, the cooling of the strip is performed instable mode, on account of the presence of a vapor film at the surfaceof the strip. Said temperature L is the temperature referred to asLeidenfrost. From said temperature L and until the strip reaches thetemperature marked by the letter N on the X-axis, the cooling of thestrip takes place in a transition mode having an unstable vapor film.The exchange coefficient thus increases significantly on account of therupture of the layer of insulating vapor. Subsequently, from thetemperature N until the end of the cooling, this is performed in anucleate boiling regime.

The Leidenfrost temperature is a critical point which depends onnumerous parameters, in particular the features of spraying, such as thesurface density of water projected, the speed and the diameter of thedrops, the mesh size of the nozzles, the distance of the nozzles fromthe strip, the temperature and the type of the fluid. Said parametersmay be determined experimentally for different types of spraying nozzlesin order to form tables which are applicable to cases of industrialproduction. The typical values of Leidenfrost temperature are between200° C. and 700° C., depending on the effectiveness of the cooling. Anexperimental database makes it possible to know the determination of theLeidenfrost temperature associated with each case of production of theline.

With reference to the diagram of FIG. 2 of the accompanying drawings, anenlarged view of the section 104 for rapid cooling by means of anaqueous liquid solution, of FIG. 1 , is visible. It comprises, at theinlet, a means 5 for atmosphere separation, making it possible toprevent the reducing atmosphere of the section 103 of slow gaseouscooling, located upstream, from being polluted by the water vapororiginating from the section 104.

In said section 104, the cooling of the strip is performed by projectionof a liquid thereon, or of a mixture of a liquid and a gas, by means ofnozzles 3 arranged on either side of the strip. Said cooling sectioncomprises two zones 109, 110 which are located on two different strandsof the strip. In the example shown, the first strand is falling and thesecond is rising. In a variant, the first strand could be rising, andthe second falling.

On the falling strand of the zone 109, the cooling of the strip isperformed using water, or using a mixture of water and a gas. Saidcooling in the zone 109 makes it possible to bring the strip to atemperature substantially equal to the Leidenfrost temperature. On therising strand of the zone 110, the cooling is performed by a liquidsolution which is non-oxidizing for the strip and is stripping for theoxides present at the surface of the strip, or by a mixture of saidliquid solution and a gas.

An atmosphere separating airlock 5 is arranged on the horizontal strandpositioned between the falling strand of the zone 109 and the risingstrand of the zone 110. Said airlock prevents the vapors of thenon-oxidizing liquid of the zone 110 from entering the zone 109 and frompolluting the water vapor present in said zone. The vapors extracted insaid airlock may be condensed, and the liquid obtained may be returnedinto the circuit for recirculation of the cooling liquid of the zone110.

Upstream of the airlock 5, gas knives 15 make it possible to limit theamount of water brought in by the strip from the zone 109 into the zone110. Said gas knives 15 blow a gas onto the strip at high speed, inorder to expel the water present thereon. By limiting the entry of waterfrom the zone 109 into the zone 110, the dilution of the liquid solutionused in section 110 is limited.

Each zone 109, 110 comprises a collecting tank 16 which makes itpossible to collect the stream of water from the zone before returningit towards the nozzles of the zone using means which are not shown, inparticular a pump.

A means 5 for atmosphere separation preceded by gas knives 15 ispositioned at the output of the liquid cooling section 104. These makeit possible to prevent the reducing atmosphere of the downstreaminduction heating section 105 from being polluted by the vapororiginating from the section 104, or by the water carried along by thestrip.

With reference to the diagram of FIG. 3 of the accompanying drawings, arepresentation of the section 104 for rapid cooling according to avariant of the invention is visible. Therein, the two zones 109, 110 arearranged on the same strip strand. Said strand is falling in the exampleshown, but it could also be rising.

The induction heating section 105 comprises an inductor 2 intended forreheating the strip. The overaging section 106 comprises other means foratmosphere separation 5, arranged at the inlet and at the outlet of saidsection.

The means for atmosphere separation make it possible to have differentatmospheres in each section. Thus, for example, the atmosphere of theoveraging section 106 may contain 20% hydrogen, while the atmospheres ofthe sections arranged upstream and downstream contain just 4%. The meansfor atmosphere separation may be of the roller type, having one singlepair of rollers positioned face-to-face on either side of the strip orfurther. Advantageously, they comprise two pairs of rollers, and tappingis performed between the two pairs of rollers in order to increase theeffectiveness of the atmosphere separation.

The furnace outlet section 107 comprises a gaseous cooling chamber inthe rising strand and an inductor 6 in the falling strand. Depending onwhether the overaging temperature is higher or lower than thetemperature of immersion of the strip into the coating bath 7, the stripis either cooled in the cooling chamber or heated by the inductor 6.

With reference to the diagram of FIG. 5 of the accompanying drawings, aschematic enlarged view of FIG. 1 showing the sections 105 for inductionheating and 106 overaging in greater detail can be seen. Said twosections comprise injection points 10, 12 and exhaust points 11, 13 forthe gaseous mixture forming the atmosphere of said sections. Theoveraging section comprises means 8, 14 for heating the strip, whichmeans are intended to bring the strip, or the film at the surface of thestrip, to a temperature sufficient for starting the chemical reactionsof residue reduction, in particular when the overaging temperature isnot sufficient for this. The heating means 8 is for example radiative orby induction. It is selected from those which allow for significanttransfer of heat to the strip over a short length. Indeed, it must makeit possible to rapidly bring the strip to the temperature necessary forstarting the chemical reactions in such a way as to limit the dwell timeof the strip at a temperature above the overaging temperature. Theheating means 14 is convective. It consists in blowing a gas, at a hightemperature, onto the strip, for example at 800° C. The overagingsection may comprise just one single means 8, 14 for heating the strip.If it comprises both, the heating means 8 may be positioned downstreamof the heating means 14, in the direction of travel of the strip, asshown in FIG. 5 , or upstream thereof.

The overaging section also comprises a means 9 for cooling the strip,making it possible to rapidly return the strip to the overagingtemperature.

With reference to the graphs of FIGS. 6 to 12 of the accompanyingdrawings, it is possible to see examples of thermal cycles of the stripas a function of time, according to examples of applications of themethod according to the invention, shown schematically. In these graphs,the temperature of the strip is on the Y-axis, and the time is on theX-axis. For all these examples, the same strip format and the same speedof travel of the strip will be considered. The curves of these graphsstart with a stage illustrating the end of the maintenance M, at atemperature TM, in the section 102, followed by a slow gaseous coolingRL, to a temperature TRL, in the section 103, then rapid liquid coolingRR in the section 104, to a temperature TRR, an overaging O, at atemperature TO, in section 105.

In the example of FIG. 6 , the steel grade and the metallurgicalstructure sought do not require the strip to be cooled to below theoveraging temperature. In the same way, they lead to an overagingtemperature TO which is sufficient for starting the chemical reactionsof residue reduction, and the length of the overaging section is suchthat the dwell time of the strip in the overaging section is sufficientfor eliminating the residues. The strip is cooled in the section 104 tothe overaging temperature TO, and it is kept at said temperature in theoveraging section 106 by the heating means of the section, for exampleradiant tubes. The induction heating section 105 is not involved. Theatmosphere of the overaging section has a hydrogen content suitable forsaid steel and for the operating conditions. It is for example 10% for4% in the upstream 105 and downstream 107 sections.

In the example of FIG. 7 , the steel grade and the metallurgicalstructure sought require the strip to be cooled to a temperature TRR ofless than the overaging temperature. The overaging temperature TO isstill sufficient for starting the chemical reactions of residuereduction, and the length of the overaging section is such that thedwell time of the strip in the overaging section makes it possible toeliminate the residues. The strip is cooled in the section 104 to thetemperature TRR. The inductor of the heating section 105 makes itpossible to raise the temperature of the strip back to the overagingtemperature TO, and it is kept at said temperature in the overagingsection 106 by the heating means of the section.

In the example of FIG. 8 , the steel grade and the metallurgicalstructure sought lead to an overaging temperature which is insufficientfor starting the chemical reactions of residue reduction. However, theydo not require the strip to be cooled to below the temperature TEnecessary for starting the chemical reactions. The strip is cooled inthe section 104 to the temperature TE, for example 400° C. The inductionheating section 105 is not involved. The stage E at the temperature TEis limited to the period required for starting the chemical reactions,for example one minute. Depending on the speed of travel of the strip,said stage may be obtained during the passage of the strip into theinduction heating section 105, the thermal insulation of which preventscooling of the strip. If the dwell time of the strip in the inductionheating section is not sufficient, the stage E ends at the start of theoveraging section. Cooling RE is then performed, in order to bring thestrip to the overaging temperature TO. Depending in particular on theformat of the strip, the speed of movement thereof, and the temperaturedifference between TE and TO, the cooling may be obtained simply bycontrolling, in particular stopping, the heating equipment arranged atthe inlet of the overaging section. If this is not sufficient, a coolingmeans 9 makes it possible to cool the strip to the temperature TO. Saidmeans consists for example in blowing, onto the strip, a gas at anappropriate temperature. The strip is then kept at the overagingtemperature by the heating means of the section.

In the example of FIG. 9 , the steel grade and the metallurgicalstructure sought lead to an overaging temperature which is stillinsufficient for starting the chemical reactions of reduction. Moreover,they require the strip to be cooled to below the temperature TEnecessary for starting the chemical reactions. The strip is thus cooledin the section 104 to the temperature TRR. The induction heating section105 is involved for reheating the strip to the temperature TE. Onceagain, the stage E at the temperature TE is limited to the periodrequired for starting the chemical reactions.

In the example of FIG. 10 , the steel grade and the metallurgicalstructure sought require the strip to be cooled to below the overagingtemperature TO. Moreover, they lead to an overaging temperature which isinsufficient for starting the chemical reactions of reduction when saidtemperature TO is reached by the induction heating of the section 105.The strip is cooled in the section 104 to the temperature TRR. Theinductor of the heating section 105 makes it possible to raise thetemperature of the strip back to a temperature TI which is less than theoveraging temperature TO. After said first heating CI, a second heatingCC makes it possible to bring the strip to the overaging temperature.Said heating CC is performed by blowing a hot gas onto the strip, forexample at 800° C., by the means 14 visible in FIG. 2 . This leads to atemperature of the film at the surface of the strip that is at leastequal to the temperature TE necessary for starting the chemicalreactions of residue reduction, while the core of the strip may remainat a lower temperature. It is thus not necessary to bring the strip to atemperature above the overaging temperature in order to start thesereactions.

The example of FIG. 11 is close to that of FIG. 10 . It is distinguishedtherefrom in that the overaging temperature TO is substantially lower.The second heating CC is also performed by blowing a hot gas onto thestrip by the means 14. This leads to a temperature of the film at thesurface of the strip that is at least equal to the temperature TEnecessary for starting the chemical reactions of residue reduction,while the core of the strip only reaches a temperature TS lower than TE,but in this case said temperature is greater than the overagingtemperature TO. As in the example of FIG. 8 , cooling RE is thenperformed, in order to bring the strip to the overaging temperature.

FIG. 12 shows three examples of thermal cycles according to theoveraging temperature for a galvanizing line. The first example shown ina solid line corresponds to the case of FIG. 6 , i.e. that the overagingtemperature T01 is lower than the temperature TI at which the strip mustbe immersed in the coating bath. The strip is heated from T01 to TI inthe furnace outlet section 107 by means of the inductor 6. For these 3examples, after leaving the bath, cooling FC brings the strip to ambienttemperature. The second example shown in a broken line corresponds tothe case where the overaging temperature T02 is equal to the temperatureTI at which the strip must be immersed in the coating bath. The stripsimply passes through the furnace outlet section 107 without beingeither heated or cooled. The third example shown by a succession ofcrosses corresponds to the case where the overaging temperature T03 ishigher than the temperature TI at which the strip must be immersed inthe coating bath. The strip is cooled from T03 to TI in the furnaceoutlet section 107. This cooling is performed by blowing a gas onto thestrip, for example a mixture of nitrogen and hydrogen.

Of course, the invention is not limited to the embodiments describedabove, and a number of developments can be made to said embodiments,without departing from the scope of the invention. Moreover, the variousfeatures, types, variants, and embodiments of the invention may beassociated with one another, in accordance with various combinations,insofar as they are not mutually incompatible or exclusive.

1. Method for rapid cooling of a metal strip travelling in a continuousline, performed in a section of said line, and for removing residuesformed during said rapid cooling in a section of said line,characterized in that it comprises a first step of water cooling, orcooling using a mixture of water and a gas, followed by a second step ofcooling using a liquid solution that is non-oxidizing for the strip andis stripping for the oxides present at the surface of the strip, orusing a mixture of said liquid solution and a gas, said second stepleading to the presence of residues at the surface of the strip,followed by a step of removing said residues obtained by reduction ofsaid residues by means of hydrogen.
 2. Method according to claim 1,wherein the first step of cooling cools the strip to a temperature ofgreater than or equal to the Leidenfrost temperature.
 3. Methodaccording to claim 1, wherein the second step of cooling cools the stripfrom a temperature of less than or equal to the Leidenfrost temperature.4. Method according to claim 1, wherein the step of removing residues isperformed when the metal strip is at a temperature of between 50° C. and600° C., and for a period of between 15 seconds and 300 seconds. 5.Method according to claim 1, wherein the step of removing residues isperformed when the metal strip is in an atmosphere of which the hydrogencontent is between 5% and 100% by volume, and preferably greater than orequal to 10%.
 6. Method according to claim 1, further comprising a stepof pre-oxidation, or of selective internal oxidation, of the surface ofthe metal strip, performed in a preheating section of the strip or aheating section of the strip, or a temperature maintenance section ofthe strip, said section being arranged upstream of the section of rapidcooling of the strip, according to the direction of travel of the strip.7. Method according to claim 1, implemented on a continuous line havinga section for dip coating of a metal strip in a molten bath, furthercomprising, after the step of removing residues, a step of heating ofthe strip or a step of cooling of the strip, in order to bring the stripto a temperature close to the temperature of the bath.
 8. Continuoustreatment line for a metal strip, comprising a section for rapid coolingof the strip and a section for removing residues formed during thecooling of the strip using a liquid solution that is non-oxidizing forthe strip and is stripping for the oxides present at the surface of thestrip, or using a mixture of said liquid solution and a gas, said rapidcooling and residue removal sections being capable of implementing amethod for cooling and for removing residues according to claim
 1. 9.Line according to claim 8, wherein the section for removing residuescomprises, at the inlet in the direction of travel of the strip, a rapidheating device for bringing the strip to a temperature close or equal toa predetermined temperature at which chemical reactions for reducingresidues start.
 10. Line according to claim 8, wherein the section forremoving residues forms part of an overaging section.
 11. Line accordingto claim 8, wherein the section for removing residues comprises a meansfor blowing hydrogen, or a hydrogenated atmosphere, onto the metalstrip.
 12. Line according to claim 11, further comprising a chamber forpre-oxidation, or selective internal oxidation, of the surface of thestrip arranged in a preheating section, a heating section, or atemperature maintenance section, of the metal strip, said section beingpositioned upstream of the section of rapid cooling, in the direction oftravel of the strip.
 13. Computer program comprising instructions whichlead a continuous treatment line to execute the steps of a methodaccording to claim 1; wherein the continuous treatment line comprises asection for rapid cooling of the strip and a section for removingresidues formed during the cooling of the strip using a liquid solutionthat is non-oxidizing for the strip and is stripping for the oxidespresent at the surface of the strip, or using a mixture of said liquidsolution and a gas, said rapid cooling and residue removal sectionsbeing capable of implementing a method for cooling and for removingresidues.